Efficiency vs. Flow
The tradeoff between filtration efficiency and throughput will always be with us. Andrew Zydney, Ph.D., who heads the department of chemical engineering at the Pennsylvania State University, has discovered a way to diminish that compromise. Dr. Zydney uses electrically charged UF membranes that repel proteins of similar charge but simultaneously provide the high flux expected from filters with larger pores. Charged membranes are currently used industrially to recover electropaints, charged-particle pigments used in automobiles and appliances.
Final product formulators will be interested in this charge-directed filtration since their product streams are relatively pure. Processors prefer to perform these purifications rapidly—in three hours or less—with as low a membrane area as possible, to avoid product degradation. “The normal way to achieve this is with tight membranes, which have low flux and require long processing times,” states Dr. Zydney.
Charged ligands consist of quaternary amines and a variety of counter-ions. Dr. Zydney continues to investigate the impact of varying the ligands, charge density, length of the spacer arms connecting the charged ligand to the membrane material, and even the counter-ions. The standard negative ion for quaternary amines is hydroxide, but evidence from chromatography suggests the counter-ion may affect selectivity. “Whether this holds here or not remains to be seen,” says Dr. Zydney
Dr. Zydney has been testing his invention in the real world through collaborations with Genentech (www.gene.com) and Millipore. “A commercial, first-generation product is in the works, but I don’t know the timeline,” he says
Although some newer processes are shrinking as cell productivity and protein activity rises, existing processes, especially those for approved monoclonal antibodies, are growing in scale as the number of approved indications and market demand increase. For Mabs, that means bioreactors of 20,000 liters or more, this has created the need for higher-capacity and -flux culture filtration.
The market is also shifting from the use of 0.2-micron filters to 0.1-micron to better protect against mycoplasma contaminations, according to Ralf Kuriyel, director of biopharm applications R&D at Pall (www.pall.com). In the absence of industry standards for 0.1 micron rated filters, the Parenteral Drug Association (www.pda.org) has formed a task force to establish standard challenge conditions for vendor qualification of mycoplasma retentive filters, which are nominally rated at 0.1 micron.
In this particular case, the task force must balance the requirements of high throughput against retention of mycoplasma—essentially a safety tradeoff. Setting retention standards too high, for example at 100%, would dramatically degrade performance. The question is how much less than 100% retention of pathogenic bacteria will regulators accept?
High titers, although always welcome, are somewhat of a mixed blessing since they present challenges for post-harvest clarification in the form of increased debris. The filtration following protein capture has also become more difficult due to the precipitation occurring during product pool neutralization. “Biotech manufacturers are looking for a single filter that has a high capacity for clean streams, such as buffers, and can also accommodate streams with higher solids loading,” says Kuriyel.
Similarly, biomanufacturers are adopting small virus filtration early in bioprocesses, with a premium on high flux, capacity, and log removal values whose performance does not degrade as a function of throughput, even with challenging process fluids. “Users,” says Kuriyel, “prefer to conduct virus filtration at protein concentrations between 15–25 mg/mL, a target that may require the development of new membranes.”
As the final protein concentrations reach 10 to 15 g/L, the final ultrafiltration/diafiltration step becomes difficult due to higher viscosities, necessitating design of products with reasonable feed channel pressure drops and adequate mass transfer properties. “Disposable TFF devices are attracting more interest for these applications,” says Kuriyel, “due to the simplicity of operation and validation.”
In March, Pall introduced the Supor® UEKV and Fluorodyne® EX high-flow, sterilizing-grade cartridge filters. The 0.2-micron Supor UEKV combines Pall’s Ultipleat® membrane packing technology with a small-core design, providing a 50% increase in membrane area in the same size filter element. Supor UEKV also incorporates Pall’s machV polyethersulfone (PES) prefilter membrane layer with an asymmetric pore structure.
Also utilizing the Ultipleat design, Fluorodyne EX features a built-in mach V PES prefilter over a high-flow 0.2-micron polyvinylidinedifluoride sterilizing membrane. The ability to steam sterilize both filters without pre-wetting reduces preparation time.